Port to new tp-glib client factory

- EmpathyChannelFactory has been changed to EmpathyClientFactory and inherit
from TpAutomaticClientFactory.
- We now always use the _with_am variant of TpSimple* constructors
- We define our own factory as default.
- Replace empathy_get_account_for_connection() by tp_connection_get_account()
- The factory is passed to EmpathyTpChat and TpyCallChannel
- Use tp_simple_client_factory_ensure_account() instead of
tp_account_manager_ensure_account().
- Rely on the factory to prepare connection features.

This should ensure that all the TpProxy and TpContact objects created in
Empathy are shared and use EmpathyClientFactory.

https://bugzilla.gnome.org/show_bug.cgi?id=655799